'Necessary & Sufficient' Cover the Safety Level

Engineers need to wrestle with a wide variety of analytical and computational tools to solve complex engineering problems, such as designing a turbine blade that can withstand turbulent hot fluids, and designing a control system for an autonomous robot that can tread through a forest. But to solve most of the simpler engineering problems, a good understanding of basic principles of engineering and good judgment prove to be more useful than any sophisticated problem-solving tools.

There are many basic principles and maxims that engineers can use to solve simple problems. One such trick is to ask at least two questions. First, what is necessary to satisfy the requirement? Second, what is sufficient to satisfy the requirement? You will have a good, acceptable design if you incorporate answers to these two questions into your design. The questioning not only helps in designing but also in setting the specifications.

A necessary condition is the mandatory condition that the design should satisfy to meet the requirement. Examples of necessary conditions include: the rivet must support a shear stress of at least 150 MPa; the bottle must withstand a pressure of at least 80 psi; and the resistor must have a power rating of at least 1W.

A sufficient condition is the level of safety that's sufficient for the design. Examples of sufficient conditions include: the bridge design that can carry 100 tons of load must have a sufficient safety level; the container design that can withstand a 200C temperature must have a sufficient safety level; and the resistor with a 5W power rating must have a sufficient safety level.

If you observe closely, the relationship between necessary and sufficient conditions is "factor of safety," which is discussed in most engineering books. Necessary condition is obtained by applying basic engineering principles, and sufficient condition is obtained by multiplying the necessary condition with a factor of safety. It's often the engineer's experience and judgment that plays a key role in setting the numerical value for the factor of safety.

The questioning is also useful in setting the specifications for a system. For example, it's necessary for an information system to respond to a request for an approval of a credit transaction in three seconds and it's sufficient if it can respond in 0.5 seconds. The three-second necessary condition is based on the maximum time a customer will wait for the approval before losing his patience. The 0.5 second sufficient condition is based on the fastest computing system that can be used for the application without breaking the bank. Note that in this example, the sufficient condition is obtained by dividing (instead of multiplying) the necessary condition by the factor of safety.

The same principle can be applied in your personal life. Before going on a road trip for a long weekend, ask yourself: How much cash do I need to cover all the tolls, tips, and parking meters, and how much cash is needed to provide a level of safety that's sufficient?

— Raghavendra Angara, PhD, is a senior mechatronics R&D engineer. He belongs to the ASME, ISA, IEEE, and ASQ.

Great article. "Necessary" is often fairly easy to calculate knowing what is needed, but that "Sufficient" number can vary all over the place depending on perspective. I think personal experience is a variable that can have great impact on what is sufficient, so can lead to a lot of debate among peers. How safe is safe? How much is too much?

That is a good point. The sufficient condition is usually the bounding condition. The necessary condition is that which is called out specifically in the specification. Of course, this assumes the requirements are correct.

I agree. What I'm saying is that "sufficient" (as much as needed) is very subjective and based on individual perspective and opinion. My wife and I have very different views on how many diamonds are 'sufficient' for her wardrobe. One may be necessary or required to display her merital status, but we don't agree on the sufficient number.

This is an excellent post--excellent. I had a professor who always stated, " the only thing less desirous than an undersign is an over design. He was all about applying the proper safety factors to design problems and stressed that to all of his students. You were automatically "dinged" if you did not state and apply safety factors and how those affected the assembly of components.

I think we have just seen a remarkable violation of "necessary and sufficient" with programming and launch of the "Affordable Healthcare Act". A disaster due to improper planning and bogus architecture. Our country could definitely benefit from reading your post.

"It's often the engineer's experience and judgment that plays a key role in setting the numerical value for the factor of safety."

Reminds me of the chief engineer on the Enterprise:

[the U.S.S. Enterprise is being sucked into a black hole, seconds away from doom] Scotty: I'm giving her all she's got, Captain! [the bridge ceiling begins to crack as the ship's drawn closer] James T. Kirk: All she's got isn't good enough! What else ya got? Scotty: Um... Okay, if we eject the core and detonate, the blast could be enough to push us away! I cannae promise anything, though! [the viewing window starts to rupture] James T. Kirk: DO IT, DO IT, DO IT!

Probably not a scenario he would have used a computer model for...

While a work of fiction, it still speaks to the importance of factoring in an experienced engineer's judgment that may not necessarily agree with a computer model...and that "sufficient" can also depend on the situation.

The standards electrical machines and components are required to meet in the food processing industry are far more stringent than those in traditional plant construction. For specialized production environments such as these, components must not only resist thermal and physical stresses, but they must also be resistant to the chemicals used to sterilize equipment.

The word “smart” is becoming the dumbest word around. It has been applied to almost every device and system in our homes. In addition to smartphones and smart meters, we now hear about smart clothing and smart shoes, smart lights, smart homes, smart buildings, and every trendy city today has its smart city project. Just because it has a computer inside and is connected to the Web, does not mean it is smart.

Was Steve Job’s signature outfit of a black turtleneck, jeans, and sneakers the secret behind his success? Maybe, or maybe not, but it was likely an indication of a decision-making philosophy that enabled him to become one of the most successful innovators of all time.

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